There are two coordinate systems used to present the colors displayed on a display device. One is RGB system (Generally, RGB is used to indicate three original colors without gamma correction R′G′B′ for colors with gamma correction), and the other is Y, Cb, Cr system (or Y, U, V; or Y, Pb, Pr). However, we must use only the R′G′B′ system to display colors on a display device such as TFT-LCD.
FIG. 1 is a diagram of RGB coordinate system. In RGB coordinate system, the color distribution is presented in a cube style. Whenever one of the axes is adjusted, the color of each pixel will be affected, so, controlling over a specific color independently is very difficult. Consequently, we generally transfer the RGB coordinate system into the Y (illumination), Cb (the first tint), Cr (the second tint) system (or Y, U, V; or Y, Pb, Pr).
FIG. 2 shows the Y, Cb, Cr coordinate system. After transferring RGB into Y,Cb,Cr, each color can be simply controlled by two parameters (Cb, Cr). So, most of the video image systems use Y, Cb, Cr (or Y, U, V; or Y, Pb, Pr) coordinate system to perform the color management.
Colors shown on a generic display device can be adjusted with different users' needs. Taking adjustment of blue as an example, some may prefer magenta-oriented blue, but others may prefer cyan-oriented blue. Yet, traditional color adjustment of a display device can't successfully adjust a specific color class independently. So, when some color class is adjusted through tuning, other color classes will also be adjusted simultaneously. It will cause the incongruity in pictures of a display device. For example, if, we adjust only the blue (B) element of every pixel (Each pixel is composed of RGB.), of course the blue part will be modified, but actually all the other color elements of the pixel will be affected. Even if we use Y, Cb, Cr coordinate system and adjust Cb or Cr only, the outcome will be the same. So, traditional hue adjustment of a display device has this drawback. Because colors can't be adjusted independently, the whole image color can't be tuned to a satisfactory condition.
The theory of adjusting a specific color class is first to determine Cb and Cr values of each pixel. If we've decide to adjust some pixel's color, in what range we should adjust (ex. +5% or −5%) is the next step for consideration. However, adjusting a specific color class will lead to the problem of boundary, if pixels of other neighboring colors are also inside the area for adjustment. As FIG. 3 shows, the image colors of an orange includes orange of red class (area A) and orange of yellow class (area B). If we only adjust the pixels in the red class, there will be an obviously non-continuous rim happened between area A and area B, which causes incongruity of the image colors. Therefore, how to avoid the problem of boundary when adjusting a specific color class is exactly the concern of this invention.